In the past decade, silicon-silicon bonding has received increasing use in the fabrication of micromechanical structures and power electronics. Silicon-silicon bonding is also a candidate technology for manufacturing silicon-on-insulator (SOI) wafers, which are attractive for high speed digital microelectronics. By way of example, in metal-oxide-semiconductor (MOS) transistors fabricated on SOI wafers, the parasitic capacitance associated with source and drain junctions is minimal.
In the bonding process, the polished surfaces of the silicon wafers to be bonded are pressed together at moderate temperatures to form an initial pre-bond. To complete the bond and achieve full strength, the wafers require baking at approximately 1000° Celsius. Conventionally, this is done in large furnaces, which require long ramp times, consume large amounts of power, and have significant manufacturing footprints. The resultant high thermal budget limits process flexibility, making process integration difficult. In addition, furnaces lack the flexibility to adapt to unconventional bonding requirements such as multi-wafer stacks.
It can be appreciated that silicon bonding through electromagnetic induction heating (EMIH) overcomes many conventional limitations. Heretofore, the use of electromagnetic radiation for wafer bonding has been limited. Since it has been assumed that silicon is transparent to electromagnetic radiation because of its small imaginary dielectric response, intermediate glue layers of metal were used to absorb the radiation. In reality, however, the ohmic response of silicon to an oscillating magnetic flux can be used to directly heat the silicon.